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Rocks
Chapter 10
Rocks & the Rock Cycle
• Magma is the parent material for all rocks
• There are 3 major types
• Any type can be changed into any of the
other types through a series of changes
called the rock cycle
3 Major Rock Types
• Three major types
– Igneous = magma cools & hardens
– Sedimentary = sediment is deposited and
hardens after being compressed & cemented.
– Metamorphic = forces such as extreme heat
and pressure and chemical processes change
the form of existing rock.
The Rock Cycle
Overview
• Hardened magma cools -> igneous rock
• Igneous rock breaks down into sediments
-> sedimentary rock
• Sedimentary rock subjected to extreme
heat/pressure -> metamorphic rock
• Metamorphic rock under extreme
heat/pressure -> melt into magma (start over)
The Rock Cycle (cont)
• Rock does not always make it through
entire cycle
– Igneous rock not exposed to surface will
never become sediment – may go directly to
metamorphic
– Igneous and sedimentary may melt directly
into magma
– All 3 types may form sediments when
exposed
The Rock Cycle
Igneous Rock
• Two types = classified by where magma
cools
– intrusive = cool deep below crust
• May “intrude” into other rock layers
• Cools slowly
– extrusive = lava cooling on surface
• Cools rapidly
Igneous Rock (cont)
• Texture determined by rate of cooling
– Intrusive = cools slowly
• Course-grained texture
• Ex: granite
– Extrusive = cools rapidly
• Little time for crystal formation
• Fine-grained texture
• Ex: basalt
Igneous Rock (cont)
• Some extrusive rock cools slowly at
first then more quickly as it nears
the surface
– Produces both large and small crystals
– Porphyry
• Extremely rapid cooling produces
rock without crystals
• Gases escaping during rapid cooling
may form small bubbles in rock
Igneous Rock (cont)
• Composition of igneous rock is determined
by chemical composition of magma.
• 3 Families – based on composition
– Granite
– Basalt
– Diorite
Igneous Rock (cont)
• Granite Family
– Felsic magma – high silica
– Light color
– Main minerals = orthoclase feldspar & quartz
– Ex: granite (intrusive)
rhyolite
and
obsidian (extrusive)
Igneous Rock (cont)
• Basalt Family
– Mafic lava = low silica, high iron
– Dark color
– Main minerals = plagioclase feldspar and
augite
– Ex: basalt
&
gabbro
Igneous Rock (cont)
• Diorite Family
– Medium colored
– Main minerals = plagioclase feldspar,
hornblende, augite, and biotite.
– Little or no quartz
– Ex: diorite
&
andesite
Igneous Rock Structures
Intrusions = underground
• Batholith
• Largest (cover over 100km²)
• Form cores of many mountains
• Stock
• Similar to batholith but cover less area
• Laccolith
• Magma pushes overlying rock layer into an arc
• May form dome mountains
• Sills
• Sheet of magma flows between layers of rock (horizontal)
• Dikes
• Magma flows upward through fractures and cut across rock
layers vertically
Igneous Intrusions
Igneous Rock Structures
Extrusions = at surface
• Volcanic neck – softer parts of volcanic
cones wear away exposing solidified rock
in vent.
• Many are simply flat lava flows
• Lava plateaus – lava flows out long cracks
in the surface
• Cover vast areas
• Fill in valleys and cover hills to form flat plateaus
Igneous Extrusions
Sedimentary Rock
• Compaction and cementation form
sedimentary rock.
– Compaction – weight of overlying sediments
causes pressure that squeezes air and water
out of sediments
– Cementation – water carries dissolved
minerals through sediments which provide a
cement to hold fragments together.
• Sedimentary rocks are classified by the
kind and size of their sediments.
Sedimentary Rock (cont)
• Clastic sedimentary rocks
– Rock fragments are carried by wind, water and ice
become compacted and cemented together =
classified by size
– Conglomerate = rounded, gravel-sized pebbles
cemented by minerals
– Breccia = gravel-sized fragments (angular with sharp
edges)
– Sandstones are made up of sand-sized grains
• Major component = quartz
• Many are porous enough to allow liquids (water,
crude oil) to move through.
– Shale consists of clay-sized particles
• Usually pressed into flat, “flakey” layers
Sedimentary Rock (cont)
• Chemical Sedimentary Rock
– Forms from minerals once dissolved in water
– Minerals may precipitate out due to
temperature changes
– Evaporites form when water evaporates and
leaves minerals behind
Sedimentary Rock (cont)
• Organic Sedimentary Rock
– Forms from remains of living things
– Coal forms from dead plant remains buried
and compacted
– Limestone forms from the shells of marine
organisms that use calcite
• Chalk is limestone made of shells of one-celled
marine organisms
Sedimentary Rock Features
• Stratification = layering = occurs
when type of sediment changes
– May be due to change in river
current, sea level, etc.
– Thickness of layers (beds)
depends on how long the sediment
is deposited
– Most water-deposited layers are
horizontal
– Wind-deposited layers may exhibit
cross-bedding
– Graded bedding = different kinds &
sizes of material are deposited in
the same layer as they settle out.
Sedimentary Rock Features (cont)
• Ripple marks
– Form from action or wind or
water on sand.
• Mud cracks
– Muddy deposits dry and
shrink causing shrinking
• River flood plains, dry lake
beds
• When area floods new deposits
fill in and preserve the cracks.
Sedimentary Rock Features (cont)
• Fossils
– The remains or traces of ancient plants and animals
– Are preserved in sedimentary rock when remains are
buried
– Harder parts may remain but most often it’s only the
impression
Sedimentary Rock Features (cont)
• Concretions
– Lumps or nodules of rock with a
composition different from the main
rock body
– Precipitated minerals build up
around existing rock
– Geodes = groundwater deposits
dissolved quartz or calcite inside a
cavity and crystals form
Metamorphic Rock
• Changing from one type of rock to another
by heat, pressure and chemical processes
= metamorphism
– Minerals may change into other chemicals
– Minerals may change size, shape, or separate
into bands (layered look)
– Hot fluids in magma may change mineral
composition by dissolving some minerals and
adding others
Metamorphic Rock (cont)
• Two types of metamorphism:
– Contact metamorphism
• Hot magma is near or touching rock
• Changes structure and/or mineral composition
• May also include hot chemical fluids working through
fractures
– Regional metamorphism
• Occurs over large areas during tectonic activity
• Plates moving against each other causes tremendous heat
and pressure at the edges – causes chemical changes in
rock minerals
• Most metamorphic rock is formed this way.
Metamorphic Rock (cont)
• Classification is according to structure
– Foliated
• Visible parallel bands
• Extreme pressure flattens mineral crystals into
bands
• Minerals of different densities may separate into
bands -> alternating light and dark
Metamorphic Rock (cont)
• Common foliated rocks
– Slate
• Pressure on shale produces thin
layers that split into sheets
– Schist
• Pressure on slate – courser
grained
– Gneiss
• Intense heat and pressure on
schist -> minerals separate into
bands of different densities
Metamorphic Rock (cont)
• Unfoliated
– Rocks do NOT have bands of crystals
• Quartzite – sandstone is highly compacted
• Marble – compression of limestone